1. Field of the Invention
The present invention relates to a multilayer ceramic circuit substrate to be used as an electronic circuit part, a process for producing the same, and an electrically conductive material to be used in production of the same.
2. Description of the Prior Art
In a conventional multilayer ceramic circuit substrate for use in an integrated circuit, conductors of a high-melting metal are built in, and the internal conductors are exposed over through-holes of a surface layer of the substrate, where they are connected to surface conductors each made of a thick film comprising copper as a main component. The foregoing structure involves demerits of a high contact resistance and a weak bonding strength.
In view of the above, proposals with a view to preventing such demerits have been made, an example of which is disclosed in Japanese Patent Application Laid-Open No. 218,693/1991. A specific description will be made while referring to FIG. 3. After green sheets constituting electrically insulating layers 11 and internal conductors 12 are cofired, the exposed portions of the internal conductors 12 over through-holes 13 of the surface layer of the substrate are covered with a metallo-organic paste comprising Pt, Pd, or the like, which is then fired to form metallized layers 14, which are then coated and covered with a paste comprising Cu as a main component by printing or the like, followed by firing to form surface conductors 15 each in the form of a copper thick film.
The foregoing conventional instance is excellent as a multilayer circuit substrate, but still has room for a further improvement. For example, the firing steps are three steps: cofiring of the laminated green sheets having the internal conductors therein, firing of the metallized layers, and firing for formation of the surface conductors. Thus, a further decrease in the number of steps is desired. Furthermore, since firing involves deviations of dimensions, the internal conductors are liable to be partly in direct contact with the surface conductors to cause a failure in connection unless wide surroundings around the through hole portions are perfectly covered with the metallized layers. Thus, the wide-area metallized layers are formed in an aspect of safety to fail to completely meet the requirements for further miniaturization of wiring patterns. When chip parts are soldered onto the surface conductors, the durability of the through-hole portions is deteriorated by soldering heat to make them unusable as lands for mounting components. Accordingly, the through-hole portions are covered with protective materials, outside of which the surface conductors are extended out to solder chip parts onto the extended-out portions thereof. This also presents a problem in further miniaturization of wiring patterns.
On the other hand, Japanese Patent Application Laid-Open No. 196,696/1991 discloses a multilayer ceramic circuit substrate having an intermediate metallic layer of an alloy of at least one of Ni and Co with at least one of W and Mo between an internal conductor comprising W and/or Mo as a main component and a surface conductor comprising Cu as a main component, wherein the intermediate metallic layer is provided for the purpose of improving adhesion and electrical conductivity between the internal conductor and the surface conductor. After a laminated insulating green sheets having the internal conductor patterns are cofired, the intermediate metallic layer is formed by electroplating, electroless plating, or the like on the exposed outer surface of the internal conductors onto which surface the surface conductor is to be bonded. Thereafter, an electrically conductive Cu paste is applied onto the intermediate metallic layer by printing, and then fired to form the above-mentioned surface conductor. Accordingly, the foregoing procedure inevitably involves three steps: cofiring of the green sheets and the internal conductor patterns, formation of the intermediate metallic layer, and formation of the layer of the surface conductor to fail to decrease the number of steps as well.